The present invention refers to a method of controlling a compressor, particularly a method that prevents the piston from colliding against the valve system provided therein, as well as to a system of monitoring the position of a compressor piston, and the compressor equipped with a piston position monitoring system.
Linear-type compressors are known from the prior art and are composed of a mechanism in which the piston makes an oscillating movement and, in most cases, there is an elastic means interconnecting the cylinder and the piston, imparting a resonant characteristic to this movement, the energy being supplied by means of a linear displacement motor.
In a known solution Axe2x80x94U.S. Pat. No. 5,704,771xe2x80x94Sawafuji Electric), the stroke of the piston is primordially proportional to the level of voltage applied to the linear motor, which is of the fixed-magnet-and-moveable-coil type. In this solution the mechanism is built in such a way, that the relationship between the extent of the stroke and the diameter of the piston is large, such that the variation of the end position reached by the piston during its oscillating movement, due to variations in feed voltage and load, does not interfere significantly with the characteristics of efficiency and capacity of cooling the compressor.
In this solution the mechanism is provided with a discharge valve built in such a way that, if the piston exceeds the maximum stroke expected in its oscillating movement, for instance when the voltage applied to the motor is excessive, the piston will contact the discharge valve, and the latter will allow for some advance of the piston, thus preventing an impact against the valve-head plate.
In another known solution, the stroke of the piston is also primordially proportional to the voltage applied to the linear motor, which is of the xe2x80x9cmoveable magnet and fixed coilxe2x80x9d type (Bxe2x80x94U.S. Pat. No 4,602,174xe2x80x94Sunpower, Inc.)
In this solution the design of the mechanism does not have a mechanical limiter for the piston stroke and is not sized to bear the excess shock of the piston against the valve plate. Due to the search for a design that is more optimized in efficiency, the relationship between the stroke and the diameter of the piston is not great, which makes the performance of the compressor more dependent upon variations in the piston stroke. As an example, the process of discharging the gas takes place in a very small portion of the stroke, about 5% of the total.
Another effect that occurs in this type of compressor is the displacement of the medium point of the oscillating movement, having the effect of displacing the piston away from the discharge valve. This is due to the elastic deformation of the resonant mechanical system formed by the piston and a spring, when there is difference in pressure between the two sides of the piston. This displacement of the medium point of the oscillating movement is proportional to the difference in pressure between the discharge and suction.
For the above reasons, in this solution, it is necessary to use a controller to control the piston stroke. The controller controls the voltage applied to the linear motor based on re-fed information concerning piston position, basically estimated from the information of current supplied to the motor and the voltage induced in the terminals of the motor (Cxe2x80x94U.S. Pat. Nos. 5,342,176, 5,496,153, 5,450,521, 5,592,073).
Another procedure employed for providing re-feed to this voltage controller is to observe if the shock of the piston against the valve plate, detected by means of a shock-detecting microphone or an acceleration meter (solution D), which generates a command for reduction of the voltage applied to the motor and, consequently, of the piston stroke.
In solution (A) the piston stroke is not controlled, and the design can allow variations in voltage and load, without any damage to the mechanism, but this brings limitation of efficiency to the product. In this solution too, the possible shocks of the piston against the discharge valve, even if not impairing the reliability of the product, entail an increase in noise.
In solution (C), the piston stroke is controlled by taking as a reference the estimated position of the piston, calculated from the current and voltage at the terminals of the motor, but this experiences errors due to the constructive variations of the motor, variations in temperature and in load, thus hindering a more precise control, which limits the efficiency and the operation in extreme conditions of cooling capacity.
Another drawback of this solution is that calculation of the displacement of the medium point of the oscillating movement becomes imprecise, which is basically caused by the average difference between the suction pressure and the discharge pressure and the elastic constant of the spring of the resonant system.
In solution (D) the maximum piston stroke is controlled by maintaining the voltage applied to the motor at a level right below that which causes collision, which is achieved by detecting collisions and, on the basis of the information obtained, reducing the applied voltage slightly.
The drawbacks of this solution are the collisions themselves, which are necessary for informing the proximity of the piston to the valve plate, since they cause noise and some mechanical damage, which reduces the useful life of the product.
Another disadvantage is the relatively slow reaction of this form of control, which is generally incapable of preventing collisions and reductions in the cooling capacity during periods in which there are sharp oscillations in feed voltage, that occurs often in the public electric power network.
These limitations in the more precise control of the piston stroke represent a great limitation of performance for this type of compressor. The ideal situation would be to allow the piston to come as close as possible to the valve plate, without a collision occurring. The controls known from the prior art do not permit this approximation, because there is no precision in estimating the position of the piston, and it is necessary to maintain a longer security distance, which leads the compressor not to pump gas when the discharge pressure is high, and reduces the maximum possible efficiency due to the dead volume.
The objectives of the present invention are:
to control the stroke of piston of a linear compressor, allowing the piston to advance as far as the end of its mechanical stroke, even in extreme conditions of load, without allowing the piston to collide with the valve system.
to control the stroke of the piston of a linear compressor, allowing the piston to advance as far as the end of its mechanical stroke, even in extreme conditions of load, without allowing the piston to collide against the valve system, even in the presence of extreme disturbances from the energy supply network;
to provide control over the stroke of the piston of a linear compressor, without the need for information on the displacement of the medium point of piston oscillation;
to provide control over the amplitude of the oscillation stroke of a linear compressor, permitting control over the cooling capacity developed by the compressor.
These objectives are achieved by means of a method of controlling a compressor, particularly a linear compressor, which comprises a piston and a linear motor, the piston moving along a stroke and being driven by the motor, an average voltage being applied to the motor and controlling the movement of the piston, the method comprising the steps of measuring a first movement time of the piston; comparing the first movement time with a foreseen movement time; altering the voltage if the first movement time is different from the foreseen movement time, the foreseen movement time being such that the movement of the piston will reach a maximum point.
A system for monitoring the position of the piston of a compressor is also foreseen, with a view to preventing the piston from colliding against the valve plate located at the end of the piston stroke. This objective is achieved by a system of monitoring the position of a piston, particularly a piston of a linear compressor, the piston moving along a stroke and being driven by a motor, the motor being driven by a voltage, the system comprising an electronic circuit monitoring the movement of the piston from the passage at a reference point, the reference point being located at a position farther from the end of the stroke of the piston than a maximum point, the electronic circuit measuring a permanence time that the piston remains beyond the reference point and comparing the permanence time with a desired foreseen time, the desired foreseen time being shorter or equal to a maximum stroke time of maximum stroke when the piston reaches the maximum point, the electronic circuit decreasing the voltage if the permanence time is longer than the desired foreseen time, and increasing the voltage if the permanence time is shorter than the desired foreseen time.
It is also an objective of the present invention to provide a compressor having a monitoring system that prevents the piston from advancing as far as the end of its mechanical stroke, even in extreme conditions of load, without allowing the piston to collide against the valve system. This objective is achieved by means of a compressor, particularly a linear compressor, that comprises a piston, a valve plate and a linear motor, the piston moving along a stroke and being driven by the motor, the compressor comprises an electronic circuit measuring a permanence time that the piston remains beyond a reference point and comparing the permanence time with a desired foreseen time, the desired foreseen time being shorter or equal to a maximum stroke time of maximum stroke when the piston reaches a maximum point, the reference point being located at a position farther from the valve plate than the maximum point.